1. Field of the Invention
The present invention relates to a semiconductor photosensitive element that can be easily fabricated, has a high light receiving sensitivity, and can be operated at a high speed without applying a bias voltage from the outside.
2. Background Art
A photodiode (PD) has been used to receive optical signals and obtain current signals. Particularly, in order to receive fast modulated optical signals and obtain fast modulated current signals, a reverse bias of at least 0.5 V must be applied to the PD from the outside. Since a light absorption layer is depleted by the reverse bias, and an electric field is applied to a depletion layer, electrons and holes can be rapidly transferred. By the spread of the depletion layer, the capacitance is lowered in inverse proportion to the thickness of the depletion layer, enabling higher-speed operation.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2000-164916    [Patent Document 2] Japanese Patent Application Laid-Open No. 63-127586    [Patent Document 3] Japanese Patent Application Laid-Open No. 64-59966    [Patent Document 4] Japanese Patent Application Laid-Open No. 4-252080    [Patent Document 5] Japanese Patent Application Laid-Open No. 4-230081
In some applications, optical signals transmitted through an optical fiber are photo-electrically converted in a PD to electrical signals, which are transmitted in the form of electric waves as received without passing through a power amplifier. In this case, a feed line must be provided to apply a reverse bias to the PD from the outside. However, a PD that could operate at a high speed without a reverse bias from the outside would be used to eliminate the feed line. It is conceivable, as such a semiconductor photosensitive element, to stack two PDs and use one of the PDs to generate a bias.
Although it is not for high-speed operation, a semiconductor photosensitive element wherein two or more PDs are stacked has been proposed (for example, refer to Patent Documents 1 to 5). In Patent Document 1, a semiconductor photosensitive element of pin-nip junction is described. Such a semiconductor photosensitive element is not suited to high-speed operation, because the currents in reverse directions generated in a pin-PD and an nip-PD are canceled out each other.
In Patent Documents 2 to 5, semiconductor photosensitive elements wherein pin junctions are repeatedly laminated are described. However, since the n-p junction formed between laminated pin junctions blocks the flow of DC current, PDs do not operate. Therefore, in Patent Document 4, the insertion of an ohmic layer composed of amorphous silicon, chromium, aluminum, ITO, or the like between pin junctions is proposed. However, the ohmic layer as described above cannot be crystal-grown between pin-PD laminated structures having InGaAs absorption layers used in optical communications. Even if the ohmic layer can be inserted, since the ohmic layer does not transmit light and has a high resistance, light receiving sensitivity is low, and high-speed operation cannot be realized.